/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.iotdb.library.dmatch.util;

import java.util.ArrayList;

/**
 * util for UDTFXCorr and UDTFACF
 */
public class CrossCorrelation {
    public static ArrayList<Double> calculateCrossCorrelation(
            ArrayList<Double> valueArrayList1, ArrayList<Double> valueArrayList2) {
        ArrayList<Double> correlationArrayList = new ArrayList<>();
        int length = valueArrayList1.size();
        for (int shift = 1; shift <= length; shift++) {
            double correlation = 0.0;
            for (int i = 0; i < shift; i++) {
                if (Double.isFinite(valueArrayList1.get(i))
                        && Double.isFinite(valueArrayList2.get(length - shift + i))) {
                    correlation += valueArrayList1.get(i) * valueArrayList2.get(length - shift + i);
                }
            }
            correlation /= shift;
            correlationArrayList.add(correlation);
        }
        for (int shift = 1; shift < length; shift++) {
            double correlation = 0.0;
            for (int i = 0; i < length - shift; i++) {
                if (Double.isFinite(valueArrayList1.get(shift + i))
                        && Double.isFinite(valueArrayList2.get(i))) {
                    correlation += valueArrayList1.get(shift + i) * valueArrayList2.get(i);
                }
            }
            correlation = correlation / length;
            correlationArrayList.add(correlation);
        }
        return correlationArrayList;
    }
}
